I was somewhat surprised that my earlier Dual-Fluid Depot post generated as much interest as it did. I didn’t think there were many people who cared about depots who hadn’t read any of the papers by Frank Zegler, Bernard Kutter, or myself on the topic. But I wanted to include an intro just for sake of thoroughness before tossing out this variation on the theme. Now, I’m glad I did that.
With that introductory post out of the way I wanted to share a variant on the theme that I looked into about a week ago-using the 5m diameter DCSS instead of the Centaur as a baseline for single-launch depots.
I ran the numbers for this concept, and if you:
1. Did the “depot LH2? tank as just a stretched version of the 5m diameter LH2 tank on the DCSS, using existing tooling 2. Used both the LOX and LH2 tanks from the DCSS as the “depot LOX” tanks
You could store somewhere between 100-103mT of LOX/LH2 (at a MR of ~5.8-6:1) using a depot based on existing stages and existing tooling. This would give you ~90% of the capacity of an ACES derived depot, even if ACES never gets funded.
A couple of quick notes:
1. The existing DCSS 5m LH2 tank is load-bearing. It actually connects at the bottom of the tank to the interstage, and at the top of the tank to the PLF. This means that a depot LH2 tank built using the same structure, and with no payload on top, could actually replace a good chunk of the cylindrical part of the PLF for a depot flight.
2. One of the challenges that will need to be addressed is that with the tank built into the PLF diameter, you can’t put normal MLI on the outside of the tank, since normal MLI can’t take aerodynamic loads. A deployable sunshield is one solution, but might only work at La Grange points. Another interesting one being developed by Quest Product Development Corp (in conjunction with Ball Aerospace) is LV-IMLI, an advanced version of MLI that might be able to be up to the task. A third option would be qualifying a larger diameter, “hammerhead” style fairing with say 6-7m diameter. 3. You don’t necessarily need to use both the DCSS tanks as “Depot LOX Tanks”. It is also possible to say use just the DCSS LH2 tank, and leave the DCSS LOX tank for use in storing Argon, Xenon, or some other SEP propellant. This would make more sense for L1/L2 based depots. You’d still be at somewhere around 74-75mT of LOX/LH2, but you’d also have about 28mT of liquid argon storage capacity. And you could store up to 60mT if you used Xenon instead of Argon. 4. The 100mT number was assuming a target depot O/F ratio of 6.0, which is actually probably on the lean side. The actual PLF volume is big enough to support a much bigger LH2 tank, so you could account for boiloff, etc. 5. You’d still use a CRYOTE-like module between the DCSS and depot LH2 tank to handle all of the actual depot operations functions such as power, control, rendezvous and docking, propellant transfer, etc. 6. DCSS LH2 tanks are less nice from a heat transfer standpoint because they have isogrid ribs on the inside that serve to lower the thermal resistance between the LH2 and the tank wall (more wetted surface area to conduct heat through-think of the ribs as “heating fins”), but the surface are to volume ratio of a squatter larger tank might make up for this somewhat.
Anyhow, this isn’t a thoroughly modeled idea, but it’s one way to get a 100mT capacity depot in a single launch using existing stages and existing tooling. Whether you actually need a depot this big or not is open for debate, but I wanted to point out that it was possible, even if ACES is never funded.